The present invention relates to multi-layer assemblies of thin films, such as for example, those formed from polysilicon. The thin films have predetermined stress characteristics. The present invention also relates to forming such thin films with desired stress profiles by controlling film formation conditions. By appropriately depositing variously stressed thin films and forming multi-layer assemblies therefrom, a resulting multi-layer assembly may be produced that exhibits a predetermined stress profile. The predetermined distribution of stresses in the multi-layer assembly may be sufficient, if desired, to induce geometrical effects such as curling or arcing of the assembly. A wide array of devices and applications are also disclosed that may utilize, or at least be based upon, the present invention.
Polysilicon is one of the most widely used structural materials for microelectromechanical systems (MEMS) and devices. However, when deposited by low-pressure chemical vapor deposition (LPCVD) techniques, polysilicon films typically display high residual stresses and often stress gradients as well. These stresses, particularly when compressive, may cause released devices to bend and buckle, altering their original shapes and degrading their performances. While tensile stresses may promote planarity in doubly clamped designs, such stresses also increase stiffness and cause deformation of asymmetric features. Zero-stress polysilicon thin film structures would be optimal for many applications.
Prior artisans have attempted to produce thin silicon films with reduced stress levels. U.S. Pat. No. 5,753,134 entitled xe2x80x9cMethod for Producing a Layer With Reduced Mechanical Stressesxe2x80x9d to Biebl, is directed to a method for producing a silicon layer having a reduced overall stress value, the layer being composed of two silicon sublayers. The first sublayer and the second sublayer are matched to one another such that the stresses in the two layers substantially compensate each other, and in effect, cancel each other out. However, Biebl requires that one or more auxiliary layers of silicon dioxide be provided between the sublayers. Those auxiliary layers require additional manufacturing or processing operations. Although satisfactory in some respects, a need still exists for an improved multi-layer polysilicon assembly and technique for forming, and particularly for an assembly that does not require the use of intermediate or auxiliary layers.
In addition, a disadvantage often associated with polysilicon films deposited by chemical vapor deposition techniques, pertains to the resulting relatively rough surface of the deposited film. Although approaches are known for producing films having relatively smooth finishes, typically, additional processing steps are necessary or critical process control schemes must be implemented. Accordingly, a need remains for a technique for producing films and multi-layer assemblies of such films having relatively smooth surfaces.
Recent efforts in the field of MEMS have been directed to producing microdevices and microstructures that exhibit a particular geometrical configuration. Often, it is desirable to produce curved or cantilevered geometries. However, given the relative small scale of such geometries, conventional microdevice fabrication techniques have been found to not be suitable, and in many instances, exhibit significant limitations. Accordingly, it would be desirable to provide a technique whereby microdevices and particularly microstructures, could be produced and which would exhibit a specific geometrical arrangement.
The present invention achieves the foregoing objectives and provides in a first aspect, a method for producing a multi-layer assembly of polysilicon thin films, such that the assembly has a predetermined overall bending moment. The method comprises forming a plurality of polysilicon thin film layers such that each layer exhibits a specific set of characteristics as follows. Each layer has either a devitrified microstructure and an internal tensile stress, or a predominantly columnar microstructure and an internal compressive stress. Each layer further exhibits a microstructure different than that of an adjacent thin film layer. Furthermore, each layer has a thickness and location in the multi-layer assembly such that the moments of each of the thin film layers sum to the predetermined overall bending moment.
In yet another aspect, the present invention provides a method for producing a multi-layer thin film assembly having a desired overall bending moment. The method comprises defining a total thickness for the multi-layer assembly. The method further includes defining a total number of thin film layers to constitute the multi-layer assembly. The method also includes defining a neutral plane that extends through the multi-layer assembly. The method further includes steps of forming each of the thin film layers according to a particular set of criteria as follows. Each layer has either a devitrified microstructure and an internal tensile stress or a predominantly columnar microstructure and an internal compressive stress. Each thin film layer has a microstructure that is different than the microstructure of an adjacent layer. And, each thin film layer is disposed at a location in the multi-layer assembly such that the moments of each of the thin film layers sum to the desired overall bending moment of the multi-layer assembly.
In yet a further aspect, the present invention provides a method of forming a multi-layer thin film assembly comprising a plurality of polysilicon thin films, such that the multi-layer assembly exhibits a selectively determinable overall bending moment. The method comprises steps of forming at least one first type of polysilicon thin film having either a devitrified microstructure and an internal tensile stress or a predominantly columnar microstructure and an internal compressive stress. The method also comprises steps of forming at least one second type of polysilicon thin film immediately adjacent to at least one of the first type of polysilicon thin films, such that the second type of polysilicon thin film has either a predominantly columnar microstructure and an internal compressive stress or a devitrified microstructure and an internal tensile stress. The microstructure of the second type of thin film is different than the microstructure of the first type of thin film. The method is performed such that the thin films are formed at particular locations in the multi-layer assembly such that the sum of the moments of each of the thin film layers corresponds to the overall bending moment desired for the multi-layer assembly.
In yet an additional aspect according to the present invention, a method for forming a smooth surface from a polysilicon material is provided. In this aspect, the polysilicon surface exhibits a RMS surface roughness value of less than 50 nm. The method comprises providing a substrate, and forming a polysilicon thin film having a devitrified microstructure and an internal tensile stress. The polysilicon thin film is deposited upon the substrate by chemical vapor deposition of silane at a temperature of from about 550xc2x0 C. to about 590xc2x0 C.
The present invention further provides multi-layer assemblies formed by each of the foregoing noted methods.